I urge you to consider the following in your plan... Not saying you are wrong at all, but I just want you to think of things from your daughters prospective for a moment.
If you exclude her from eating any food NOT sent from home, what happens when everyone in the class has a treat unexpectedly and she can not have it. I know almost everyone out here has experienced this from time to time and it was note easy by any means. At my brothers school, treats brought in by parents are ONLY allowed if they are in sealed boxes labeled with nutrition facts... this is to accomodate both Diabetic students who need to cover the snack with insulin, as well as children with food allergies. It may be more helpful to request something like that, and maybe to ask that they call you in the event a "special treat" shows up at school (not everyone plans ahead so it's safer to just request a call an hour or 2 before hand in my opinion). This way you can decide on a case by case basis what she can and can not have based on blood sugar readings, and other insulin on board, and all that other jazz.
Second, requiring her to eat ALL of her lunch is expected, and probably necessary, however, what if you child just doesn't FEEL like it. Do you have a picky eater. Might she behave differently away from home? What if there's something in there she decides she doesn't like? A better idea, might be (and again not saying you are wrong at all, you know your kid better than ANY of us), to pack say 4 or 5 things in her lunch. And say she needs to eat 3 of the 4 or 4 of the 5. That way, she has choices. If she doesn't feel like eating her apple then she doesn't have to, she can have carrots or whatever instead. That's sort of like going back to the old "carb choice" diet but modified a bit. Additionally, the school cannot FORCE your child to eat anything. So, you need a back up plan in case she will not eat. Maybe it's to call you so you can come up to the school and bring her home, maybe it's that the teacher has a selection of other things she can give her after calling you. You wouldn't want your precious little girl stuck at the cafeteria for hours while they tell her she HAS to eat all her food.
Keep in mind, kids WILL be kids. Friends (especially 5 y/os) are going to offer her snacks they brought, and she is probably gonna sneak some before the teacher sees if the teacher sees. That's just what they do! My brother is not allowed Ranch dressing at school, it's written right into his IEP and 504, but without fail he tries ALL the time and sometimes he does and then he gets sick and has to come home. It's a trial and error process.
A few final things, it is more than reasonable to expect she be given her insulin 20 minutes before lunch. It is more than reasonable that they would treat her lows exactly as you spell out in the 504 plan. She should never (no kid should ever) have to stay at school if she vomits or her sugars are high. I can barely think straight when my sugar gets over 250, how frustrating!!! And what a waste of time for everyone if she is sitting in class unable to think, focus, concentrate, or learn! And they should call you with ANY and ALL questions. I would suspect if they have other children in the school with D and have hired someone to help manage all of them, they are used to these type of things.
I would request 2 meetings if it were me. One maybe asap, with her teacher and the school nurse or whoever is going to be playing that role. Explain to them, and ask them what they think. If you include them, let them give ideas express concerns. This will allow them to not feel like you are coming in and "disrupting" which you worry about a bit. And who knows, they may have some ideas you haven't thought of before, or some things you need to add. How educated is the diabetes person. Does she know about carb counting, or ketones, or glucogon,maybe she does and that's fantastic. But if not, you need to know where to start. Then go back, come up with your plan/training and meet with EVERYONE the week before school.
Your daughter will do great, because she has a mom who is already excited and planning ahead.